<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="jquery.min.js"></script>
</head>
<body>

<div class="notMe">div class="notMe"</div>
<div class="myClass">div class="myClass"</div>
<span class="myClass">span class="myClass"</span>

<script type="text/javascript">
    $(".myClass").css('background','red');
</script>
</body>
</html>